Since the advent of vacuum and helium path X-ray spectrometers, and flow proportional counters, the determination of low-atomic-number elements by X-ray fluorescence has become relatively common for light-element matrices. However, there are no reports in the literature on the determination of the low atomic-number elements in a rnatrix approaching that of uranium. This paper describes the successful application of the vacuum path X-ray spectrometer for the determination of calcium in the range of 0.03 to 1.75% (on a U3O3 basis) in uranium ore concentrates. The procedure provides a 50% saving in analyst time, with no sacrifice of accuracy or precision, compared to the chemical method previously used. With a chromium target tube, the intensity of the calcium radiation is quite sufficient for accurate analyses even though the samples contain approximately 80% uranium. The heavy uranium matrix actually serves to minimize any absorption or enhancement effects that would normally be expected from the large number of impurities that are present in widely varied concentrations.

Samples are prepared by dissolving 1.8 g of concentrate in 10.0 g of Na2B4O7 at 1050°C and casting into a disk on a polished aluminum plate. The advantages of this preparation compared to the use of pelletised powders will be discussed. The Ca Kα-to-U Mγ ratio is used to minimize the1 effect of instrumental and sample variations and to provide a linear relation with respect to the calcium concentration on a U3O8 basis. Calibration standards are prepared by the addition of CaHPO4 to either pure U3O8 or a uranium ore concentrate of low residual calcium. Data obtained during routine application of this procedure will be presented.